In the mobile telephones which are operated according to the GSM standard, the standard does not permit a permanent simultaneous transmission and reception mode. These devices can therefore be operated with a common transmission and reception antenna without the serious risk of the receiver input being overloaded by the transmission signal.
The situation with mobile phones which utilize the future UMTS standard is different. In such devices having the full duplex mode, a chronological overlap between transmission mode and reception mode is permitted and occurs during operation. Therefore, in these devices—as is generally the case with the radio transceivers with chronological overlaps between transmission mode and reception mode—an efficient decoupling is necessary between the transmitter and receiver in order to avoid overloading or even destruction of the receiver input by the high power of the transmission signal.
The conventional art uses a duplex filter or a duplexer to solve the above noted problem. This comprises two high-quality bandpass filters with steep edges. At the transmission end, a transmission filter is used to suppress the transmitter noise and possible sideband emissions. At the receiver end, a reception filter ensures very high receiver selectivity. Alternatively, the aforementioned duplexer can also be embodied as a band-stop duplexer in which the transmission filter is a band stop with a zero position in the reception band, and the reception filter is a band stop with a zero position in the transmission band.
The aforementioned filter embodiments are virtually impossible to integrate owing to their specific properties and are thus not compatible with the trend for ever smaller and more lightweight mobile phones. In addition, they are costly and also problematic in terms of the ongoing reduction of costs in this field.